The present invention relates to a planetalium for projecting stars and the like with a lens, and more particularly to a planetalium having a device for appropriately shutting light projecting from the lens.
Planetaria project a star field, the sun, moon, planets and the like onto a hemispherical dome for simulating the movements of these celestial bodies. To project these celestial bodies, the planetarium has a star-field projector and projectors for individually projecting the sun, moon, stars and the like. The planetarium may also have auxiliary projectors for projecting a scene on the earth or moon.
The star-field projectors serve to project fixed stars in a celestial sphere, may be classified as pinhole star-field projectors and lens projectors. The pinhole star-field projectors have a spherical or polyhedral shell and a light source positioned at the center thereof, the shell having a number of pinholes corresponding to the fixed stars. With a celestial sphere divided into plural sectors, and the lens projectors have a plurality of projection equipments on a shell which correspond to the sectors. The projection equiement has a star-field plate having many pinholes defined therein and lenses for projecting light having passed through the pinholes onto the dome. Generally, one light source placed at the center of the shell is shared by the projection equipments.
Whatever the type, the light source radiates light in all directions, and a light shutter means is necessary to limit the range of projection to the hemispherical dome. One such light shutter means employs an opaque liquid as disclosed in U.S. Pat. No. 3,256,619. Another light shutter means comprises a shutter disposed in each projection equipment of the lens star-field projector and closable by a weight when the direction of projection by the projection equipment is oriented below the horizon. Since these light shutter means utilize the action of gravity, they are effective only for domes in which the projection limit coincides with the horizon.
In recent years, there has been a demand for planetaria for simulating starry heavens as seen not only from earth but also from space. For the simulation of starry heavens as seen from space, the horizon is substantially meaningless. Therefore, there has been proposed a dome in which the entire projecting area is composed of an inclined hemisphere and observers' seats are directed toward a dome portion which has a projecting region below the horizon.
FIG. 1 of the accompanying drawings shows such a planetarium. The planetarium of FIG. 1 has a dome 1 composed of a hemispherical portion 1a inclined to the horizon Ho by an angle of .alpha., a cylindrical vertical wall portion 1b, and a floor 1c inclined to the horizon Ho by the angle of .alpha.. The planetarium also has a star-field projector 2 disposed at the center of the hemisphere. Observers' seats 19 are arranged in the direction in which a projecting area extends below the horizon Ho. Where the projection limit is indicated by H.sub.1 and an image is to be projected through an angle .eta. in the illustrated dome, with the star-field projector 2 being a pinhole projector, any image below the projection limit H.sub.1 can be prevented from being projected by means of a cylindrical fence 20 having an upper edge lying in line with the projection limit H.sub.1, as shown in FIG. 2. However, if the star-field projector 2 is a lens projector, then the gravity-dependent light shutter mechanism cannot be employed, and an image cannot well be shielded by the fence 20 as the shielding by the fence 20 results in a varying projection limit.
Such a varying projection limit will be described with reference to FIG. 3. The star-field projector 2 has a plurality of projection equipments 3 (only one shown) disposed on a circumferential surface of a spherical shell 2a, the star-field projector 2 being rotatable in any desired direction arround the center O of the shell 2a. The star-field projector 2 also has a light source 13 disposed at the center O. Each of the projection equipments 3 is composed of a condenser lens 14, a star-field plate 15 having pinholes corresponding to stars in a star field covered by the star-field plate 15, a projection lens 16, and a casing 18 holding the lenses 14, 16 and the star-field plate 15. The projection equipment 3 has an angle of projection field: 2.omega.. It is now assumed that where the projection equipment 3 is positioned as shown with an optical axis thereof being at P.sub.1, i.e., the lower edge of the projection field angle 2.omega. coincides with the projection limit H.sub.1, the fence 20 is positioned so that it will shut off projecting light below the projection limit H.sub.1. When the star-field projector 2 is rotated clockwise in FIG. 3 from the illustrated position, the projection equipment 3 continues to project an image until the upper edge of the projection field angle 2.omega. reaches an upper edge of the fence 20, i.e., until the optical axis of the projection equipment 3 reaches P.sub.2. At the time the projection equipment 3 is positioned with its optical axis at P.sub.2, the image has been projected onto the dome while moving from an upper position toward a position H.sub.2. As a result, the projection limit has risen from the position H.sub.1 to the position H.sub.2 through an angle .theta. which is formed between a line connecting the point P.sub.2 and the upper edge of the fence 20 and the point P.sub.2 and the projection limit H.sub.1. Provided the projection field angle 2.omega. is 46.degree., the shell 2a has a radius r of 50 cm, the dome 1a has a radius R of 1280 cm, and the distance l from the center of the shell 2a to the fence 20 is 100 cm, the angle .theta. is about 20.degree. at maximum, which is quite undesirable in the performance of the planetarium. In case the upper edge of the fence 20 is aligned with the upper edge of the projection field angle 2.omega., the projection limit will fall as the star-field projector 2 rotates counterclockwise, which is also disadvantageous in the performance of the planetarium. Although the star-field projector has been described by way of illustrative example, auxiliary projectors having lenses for image projection suffer from the same problem as described above.